struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode* left, TreeNode* right) : val(x), left(left), right(right) {}
};
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {

        vector<int> res;
        if (root == nullptr) return res;
        TreeNode* cur = root;
        while (cur || !st.empty())
        {
            while (cur)
            {
                res.push_back(cur->val);
                st.push(cur);
                cur = cur->left;
            }
            TreeNode* top = st.top();
            st.pop();
            cur = top->right;
        }
        return res;
    }
private:
    stack<TreeNode*> st;
};